Catalytic Asymmetric [4+2]-Cycloadditions Using Tropolones: Developments, Scope, Transformations, and Bioactivity.

An organocatalyzed asymmetric [4+2]-cycloaddition between tropolones and electron-deficient dienophiles is presented. Complex and biologically interesting dihydrohomobarrelenone scaffolds are formed through a Diels-Alder reaction utilizing bifunctional Brønsted-base catalysis, affording the corresponding bridged bicyclic cycloadducts in up to quantitative yields with good enantio- (up to 92 % ee) and diastereoselectivity (up to >20:1 d.r.). The synthetic value of the obtained products is explored by downstream transformations, including photoisomerizations, and their biological relevancy by in vivo testing in MCF-7 cancer cells.